The goal is to determine how B cells regulate development of autoimmune diseases in a model of spontaneous autoimmune thyroiditis (SAT). All wild-type (WT) NOD.H-2h4 mice given 0.05% NaI water develop SAT, with T and B cell infiltration of the thyroid and production of autoantibody. B cell-deficient (B-/-) mice are resistant to SAT. SAT resistance is due to activity of CD4+CD25+ regulatory T cells (T reg), since B-/- mice develop SAT after transient depletion of T reg. We hypothesize that autoantigen-specific B cells are critical antigen-presenting cells (APC) for activation of effector T cells (Teff) for SAT. If B cells are absent or cannot present autoantigen, other APC preferentially activate T reg and SAT does not develop. After T reg depletion, non B cells can present antigen to Teff, and SAT develops. Anti-CD20 depletes B cells and has beneficial effects on some autoimmune diseases including SAT, but the underlying mechanisms are unclear. The proposed studies will test the hypothesis that B cell depletion by anti-CD20 in young WT mice before T eff activation inhibits SAT because T reg are preferentially activated. B cell depletion is nearly complete, and SAT remains suppressed after B cell repopulation. When anti-CD20 is given to adult mice, most marginal zone (MZ) B cells are not depleted. SAT is suppressed but continuous B cell depletion is required to maintain suppression. We hypothesize that MZ B cells produce IL-10 that plays a role in suppression of SAT in adult mice given anti-CD20. MZ B cells will be selectively depleted using mAb HMD1-5. Another aim will determine if WT and B -/- T reg differ functionally and if Teff activated by B cell APC can be suppressed by T reg. GFP+Foxp3+ cells from Foxp3 GFP knockin WT or B-/- mice will be transferred with T eff to TCR1-/- recipients to assess their ability to suppress SAT. Transfer experiments will be done to determine if activated Teff can be suppressed by T reg, and if Teff activated in the absence of B cells and T reg differ from T eff activated in the presence of B cells and T reg. Finally, the characteristics of the B cells that function as APC will be determined using bone marrow (BM) chimeras with B-/- BM and BM or B cells from mice lacking specific molecules on their B cells. NP-thyroglobulin will be used to promote uptake of thyroid antigen by NP-specific B cells to test the hypothesis that concentration of thyroid antigen by specific B cells is required for optimal activation of T eff. Insights gained in these studies will increase our understanding of mechanisms by which B cells regulate autoimmune diseases, and may provide novel strategies t modulate T reg to inhibit autoimmune diseases. PUBLIC HEALTH RELEVANCE: Drugs such as Rituximab that selectively deplete B cells are currently used to treat several autoimmune diseases in man. Although these drugs are very effective, there are large gaps in knowledge of the mechanisms by which B cell depletion inhibits autoimmune diseases, primarily because versions of the drugs able to deplete B cells in animal models were not available until recently. Our studies use a well characterized animal model of autoimmune disease to test the hypothesis that one mechanism of inhibition of autoimmune disease following B cell depletion involves activation of a subset of T cells called regulatory T cells that function to suppress autoimmune diseases. These studies will also determine if disease remission is maintained after the drug is stopped and B cells have returned, or if continued injections of the drug are required to maintain disease remission.